An excitation-physiological response delay is the time delay that occurs between excitation of excitable body tissue and a subsequent physiological response. One form of excitation-physiological response delay is the electromechanical delay (EMD) that occurs between electrical excitation of muscle tissue an the subsequent mechanical contraction. In the heart, atrial EMD is the delay between electrical activation and mechanical contraction of an atrial chamber; ventricular EMD is the delay between electrical activation and mechanical contraction of a ventricle. The inter-chamber electromechanical delay may also be defined as the delay between electrical activation of one (typically right) chamber and mechanical activation of the other (typically left) chamber. Intra-atrial EMD, as distinguished from inter-atrial EMD, can be defined as the time delay that occurs between atrial electrical activation and mechanical contraction in the same atrium. By analogy, similar definitions may be given for inter-ventricular EMD and intra-ventricular EMD. The time of electrical activation used in measuring an EMD may correspond to an intrinsic depolarization, an evoked response following a pacing pulse, or the pacing pulse.
Measurement of EMD in the heart may provide a useful diagnostic parameter (for example in classifying types of arrhythmias). EMD measurement may also be important in optimizing the synchronization of heart chambers achieved by a range of cardiac pacing therapies including cardiac resynchronization therapy (CRT). For example, measures of the intra-atrial EMD and/or inter-atrial EMD may be valuable in optimizing cardiac pacing therapy control parameters such as atrial-ventricular interval (AVI), also often referred to as the “AV delay.”